Process for treating steel, zinc, and aluminum to increase corrosion resistance



United PROCESS FOR TREATING STEEL, ZINC, AND ALU- TO INCREASE CORROSION RESIST- John A. Carroll, Philadelphia, and Nelson J. Newhard, Jr., Oreland, Pa., assignors to American Chemical Paint Company, Ambler, Pa., a corporation of Delaware No Drawing. Application April 14, 1954,

Serial No. 423,250

Claims. (Cl. 148-616) This invention relates to the art of increasing the corrosion resistance of steel, zinc or alloys thereof in which zinc is the principal or predominant ingredient and aluminum or alloys thereof in which aluminum is the principal or predominant ingredient. In the following disclosure it will be understood that alloys of either zinc or ahnninum are intended to be included where referenceiis made to either zinc or aluminum.

In many present day industries steel, zinc and aluminum are more and more frequently encountered either severally, that is, one after the other, in various manufacturingor finishing procedures or collectively in various assemblies phosphate coating processes bymeans of which a crystalline phosphate coating is chemically bonded to the surface as a result of treatment with an acidic solution. Some of these processes are capable of producing crystalline phosphate coatings on either steel or zinc surfaces without alteration of their chemical constituents while some of them are designed primarily for the treatment of steel alone and still others for Zinc alone. Similarly, in the case of aluminum, the art at present is familiar with various solutions in which fluorides are incorporated by means of which crystalline phosphate coatings can be applied to aluminum surfaces as well as to zinc or steel.

With the foregoing in mind, it is obvious that it would be highly desirable to provide a treating technique for the metals mentioned by means of which the surface of any one of these metals or of any combination of them at the same time can be successfully improved as to ultimate corrosion resistance by treatment in the same baths and by means of the same technique so that it never becomes necessary to install more than one system for the handling of all of these metals taken either singly or in combination and it is the principal object of our invention to provide such a single integrated process for the treatment of these metals by means of which their corrosion resistance can be markedly improved and their utility enhanced.

Our improved process includes two principal steps, the first being a step which employs an acidic phosphate coating solution which is designed for the production of a phosphate coating on either steel or Zinc. The second step involves the use of an acidulated solution of chromic acid and a fluorine bearing compound in accordance with the disclosure to follow. The combination of these two steps imparts excellent corrosion resistance to aluminum as well as to the steel or the zinc.

The present invention is based upon the discovery that when steel, zinc and aluminum surfaces which have been subjected to the action of an acid aqueous phosphatizing solution of a type which will produce a crystalline phosphate coating on either steel or zinc, are subsequently es atent treatment.

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treated withanacidulated solution of chromic acid and a fluorine.bearingcompound, as will furtherappear, thecorrosionresis'tance-of a so-treated aluminum surface or aso-treatedzinc surface will be considerably enhanced in vcomparis'onwith prior art procedures and, further, that a. steel. surface when so treated will be protected at least as well. as it would be had it been treated strictly in accordance with the practices of the prior art. Furthermore, this is true whether or not the aluminum surface acquires a crystalline phosphate coating from the initial Furthermore, the treatment of the present invention produces surfaces which are unusually well adapted to receivea final siccative coating of paint, lacquer, japan or the like. By the term fluorine bearing compound in the foregoing statement we mean complex fluorides such as fluosilicic acid, fluoboric acid, fluozirconic'acid,-fluostannic acid, fluotitanic acid and the soluble salts of any one of them.

In carrying out the processofour invention, as previously indicated, the metal surface is first subjected to the action of a phosphate coating solution which is capable of producing a crystalline phosphate coating on either steel or zinc. Such solutions are now so well known to the art that it is not necessary to cite specific examples. Furthermore, we have found that any such solution known to the art'is-usefulinthe process of the present invention. As purelyillustrativebut-in no way limiting, reference may be had to the phosphate coating solutions de- SCl'lbCd'illl the following ,U. S. patents: 1,911,726, 2,-

121,574, 2,132,883, 2,312,855, 2,316,811, 2,326,309. In addition. the copending application of George Schneider, Serial No. 334,379, now abandoned, assigned to the assignee of the present application, also discloses a type of phosphate coating solution which is useful for the purposes of the present invention.

After the metal has been subjectedvto the action of such a solution it is generally desirable to rinse the surface of thetreated metal with water immediately prior to subjecting it to a second treating solution according to the present invention, which solution will now be described in detail.

The amount of chromic acid and fluorine bearing compound present in the solution is important. The chromic acid concentration should be between 0.30 and 10 grams per liter and preferably between 0.4 and 3.0 grams per liter.v At a chromic acid concentration of less than 0.30 gram per liter, very little enhanced corrosion resistance is obtained. At concentrations of chormic acid greater than '10 grams perliter, any crystalline phosphate coating is apt to be at least partially stripped from the metallic surface. At concentrations between 0.4 and 3.0 grams per liter of chromic acid, maximum corrosion resistance is achieved with minimum tendency of any subsequently appliedsiccative coating to blister.

The fluorine content of the bath should be between 0.10 and 10 grams per liter, preferably between 0.1 and 1.5 grams per liter. At concentrations of less than 0.10 gram per liter, there is a marked fallingoff in the effect produced by the solution. At fluorine concentrations of greater than 10 grams per liter, there is a marked tendency to strip any phosphate coating and also to attack the surface of aluminum in a deleterious fashion. For optimum results the fluorine concentration should be between 0.1 and 1.5 grams per. liter.

The pH of the treating solution is also important. The chromic acid and fluorine bearing compounds must be present in the solution in proportions and amounts so that the pH of the solution is from 1.3 to 4.0. At pHs below 1.3, there is a tendency for any phosphate coating previously formed to be stripped from the metallic surface and at a pH greater than 4.0 very little enhanced corro.

sion resistance is imparted to the treated surface. For optimum results, the pH should lie between 1.8 and 3.0.

While treating baths. made. up. within the allowable limits indicated will yield excellent and highly useful results, we should like to point out that for best results, within the preferred limits previously specified, the following rule of thumb method of preparing the treating baths has proven very useful, i. e., when the concentration of chromic acid is low, the concentration of fluorine bearing compound should also be relatively low and, conversely, when the concentration of chromic acid is high, the concentration of fluorinebearing compound should also be relatively high.

Within the teachings just given, suitable treating solutions for this second principal step may be prepared in accordance with the following formulas;

Formula No. I

.Water, to make 1 liter.

The second treating step is effected by subjecting the surface of the metal which has already been treated with the first step to a solution of the character just described and of which typical examples are givenin Formulas 1 to 3. The length of treating time should be of such duration that the entire surface of the metal is thoroughly wetted with the treating solution. Once this wetting has taken place, the treated surface may then be dried and painted if desired.

The treating solutions of this step are useful over a wide range of temperature. In general, for concentrated treating solutions we prefer relatively low temperatures while for dilute treating solutions we prefer relatively high temperatures, i. e., 150 F. and over. However,

we are not limited to any specific range of temperature.

If desired, the second treating step may be followed by a water rinse and/or a dilute acidulated rinse of any type which is conventional in this art. Naturally, drying of the treated and rinsed surface is usual where a siccative coating is to be subsequently applied.

In conjunction with the above given examples and disclosures, it should be noted that as a source of fluorine We prefer to use soluble fluorides of zirconium and titanium. The reason for this is that treating baths prepared 'from these materials are immediately ready for use upon preparation of the bath. If the source of fluorine is a borofluoride, fluosilicate, etc., usually for optimum results, it is desirable to allow the baths to age for several hours before use. Additionally, it should be pointed .out that articles, after they have been subjected to the action of a phosphatizing solution, should usually be given a water rinse prior to their final treatment. By doing this, the pH of the final treatment bath may be maintained within their optimum pH range for a longer period of time than would otherwise be the case and, additionally, adverse effects by any foreign contaminants would be minimized.

We claim:

1. In the art of increasing the corrosion resistance of metal surfaces from the class consisting of steel, zinc and aluminum, the process which consists in subjecting the surface to the action' of a phosphatizing solution which is capable of producing a chemically bonded crystalline phosphate coating on at least one of the metals from the class which consists of steel and zinc, water rinsing the surface so treated, and then subjecting the rinsed surface to the action of an acidulated solution of chromic acid and a fluorine bearing compound, the fluorine bearing compound being chosen from the class of complex fluorides which consists of fluosilicic acid, fluoboric acid, fluozirconic acid, fiuostannic acid, fluotitanic acid and the soluble salts of any of them, the chromic acid concentration lying between 0.30 and 10 grams per liter, the fluorine content between 0.10 and 10 grams per liter with the proportions of both being so chosen as to yield a solution the pH of which lies between 1.3 and 4.0.

2. The method of claim 1 wherein the concentration of fluorine is relatively low when the concentration of chromic acid is low and relatively high when the chromic acid is high.

3. The process of claim 1 wherein the fluorine content lies between 0.1 and 1.5 grams per liter.

4. The process of claim 1 wherein the pH lies between 1.8 and 3.0.

5. .The process of claim 1 wherein the fluorine content lies between 0.1 and 1.5 grams per liter and the pH lies between 1.8 and 3.0. Y

6. A process according to claim 1 wherein fluoride of zirconium is the fluorine bearing compound.

7. A process according to claim 6 wherein the concentration of fluorine is relatively low when the concentration of chromic acid is low and relatively high when the chromic acid is high.

8. A process according to claim 6 wherein the concentration of fluorine lies between 0.1 and 1.5 grams per liter.

9. The process of claim 6 where the pH lies between l.8 and 3.0.

10. The process of claim 6 wherein the fluorine content lies between 0.1 and 1.5 grams per liter and the pH lies between 1.8 and 3.0.

11. A process according to claim 1 wherein fluoride of titanium is the fluorine bearing compound.

12. A process according to claim 11 wherein the concentration of titanium is relatively low when the concentration of chromic acid is low and relatively high when the chromic acid is high.

13. A process according to claim 11 wherein the concentration of titanium lies between 0.1 and 1.5 grams per liter.

14. The process of claim 11 wherein the pH lies between 1.8 and 3.0.

15. The process of claim 11 wherein the titanium content lies between 0.1 and 1.5 grams per liter and the pH lies between 1.8 and 3.0.

References Cited in the file of this patent UNITED STATES PATENTS 2,276,353 Thompson Mar. 17, 1942 2,516,008 Lum July 18, 1950 2,563,430 Spruance Aug. 7, 1951 FOREIGN PATENTS 675,334 Germany May 6, 1939 632,090 Great Britain Nov. 16, 1949 

1. IN THE ART OF INCREASING THE CORROSION RESISTANCE OF METAL SURFACES FORM THE CLASS CONSISTING OF STEEL, ZINC AND ALUMINUM, THE PROCESS WHICH CONSISTS IN SUBJECTING THE SURFACE TO THE ACTION OF A PHOSPHATIZING SOLUTION WHICH IN CAPABLE OF PRODUCING A CHEMICALLY BONDED CRYSTALLINE PHOSPHATE COATING ON AT LEAST ONE OF THE METALS FROM THE CLASS WHICH CONSISTS OF STEEL AND ZINC, WATER RINSING THE SURFACE SO TREATED, AND THEN SUBJECTING THE RINSED SURFACE TO THE ACTION OF AN ACIDULATED SOLUTION OF CHROMIC ACID AND A FLUORINE BEARING COMPOUND, THE FLUORINE BEARING COMPOUND BEING CHOSEN FROM THE CLASS OF COMPLEX FLUORIDES WHICH CONSISTS OF FLUOSILICIC ACID, FLUOZIRCONIC ACID, FLUOSTANNIC ACID, FLUOTITANIC ACID AND THE SOLUBLE SALTS OF ANY OF THEM, THE CHROMIC ACID CONCENTRATION LYING BETWEEN 0.30 AND 10 GRAMS PER LITER, THE FLUORINE CONTENT BETWEEN 0.10 AND 10 GRAMS PER LITER WITH THE PROPORTIONS OF BOTH BEING SO CHOSEN AS TO YIELD A SOLUTION THE PH OF WHICH LIES BETWEEN 1.3 AND 4.0 